Current limiting static switch

ABSTRACT

Disclosed is a bidirectional current limiting static circuit breaker including four power thyristors arranged to form two conducting paths. Each path includes two of the thyristors and a common current limiting inductor. Commutation means are provided to render any fault current carrying power thyristors nonconductive upon command. The current limiting inductor acts to insure that commutation proceeds to a successful conclusion once begun. The breaker also includes means for suppressing commutation induced transients and for protecting nonconducting power thyristors from externally originated voltage surges.

United States Patent Pollard 51 Mar. 27, 1973 CURRENT LIMITING STATICSWITCH 3,558,983 1 1971 Steen .317 33 sc 75 Inventor: Ernest M. PollardCherr l-l'll, NJ. l y l Primary Examiner-W1Iliam M. Shoop, Jr. [73]Assignee: General Electric Company Assistant ExaminerHarvey Fendelman[22] Filed: Jan. 21, 1972 Att0rney -J. Wesley Haubner et al.

[2i] Appl. No.: 219,621 [57] ABSTRACT Disclosed is a bidirectionalcurrent limiting static cir- 52 U.S. Cl ..317/20, 317/335 c, 317/23, witbreaker including four Power thyristors arransfid 323/24 307 252 p 307252 Q, 3 7 5 to form two conducting paths. Each path includes two [5 1]Int. Cl. ..H02h 3/08 of the thyristors and a common current limitinginduc' [58] Field of Search .317/33 sc, 20, 23, 50; Commutatim' meansare Pmvided to render 307/252 M 252 N 252 P 252 Q fault current carryingpower thyristors non-conductive 321/145 0 upon command. The currentlimiting inductor acts to insure that commutation proceeds to asuccessful con- [56] References Cited clusion once begun. The breakeralso includes means for suppressing commutation induced transients andUNITED STATES PATENTS for protecting nonconducting power thyristors fromexternally originated voltage surges. 3,102,226 8/1963 Borkovitz..323/24 X 3,133,209 5/1964 Greenwood ..307/252 M 9 Claims, 1 DrawingFigure SURGEOETECV' MEANS CONTROL C/RCU/T 14.6. SO0E65 CURRENT LIMITINGSTATIC SWITCH BACKGROUND AND OBJECTS OF THE INVENTION This inventionrelates to forced commutated static switches which are adapted to beconnected to an electric power circuit for selectively permitting orblocking the flow of alternating current therein. More particularly,this invention relates to an AC switch composed of serially connectedthyristors, associated commutation circuits selectively operative forinterrupting current through the switch, means for expediting thecommutation process, and voltage surge suppression means.

In the art of electric power distribution and utilization, it is acommon practice to employ switches or circuit breakers in order toinitiate or terminate the flow of load current on command from a controlcircuit. These switches may advantageously be constructed of solid statecontrollable switching devices such as thyristors. A silicon controlledrectifier (SCR) is one type of thyristor useful in such switches. Sincethyristor switches do not utilize any .moving parts for circuitcompletion or interruption, they are known in the art as staticswitches. Static switches may be provided with overcurrent protectivemeans to enable them to interrupt the flow of load current in responseto a sensed overcurrent of a preselected magnitude.

As is well known, an SCR comprises a body of semiconductor materialhaving a plurality of layers of alternately P and N type conductivitieswhich form a plurality of back-to-back rectifying junctions therein. Thesemiconductor body is disposed between a pair of main electrodes, oneknown as the anode and the other known as the cathode. Thyristorsadditionally include some form of gating means (e.g., in a conventionalSCR it is the gate electrode) which is operative for initiating currentconduction between the anode and cathode. When connected to a source ofvoltage and a load, an SCR will ordinarily block appreciable currentflow between its anode and cathode until triggered or fired by a signalto its gate electrode at a time when its anode is biased positive withrespect to its cathode, whereupon it abruptly switches to a relativelylow resistance conductive state. Once conducting, the SCR will continueto conduct load current even if no further triggering is provided, solong as the magnitude of current is above the predetermined holdinglevel. When the magnitude of current drops below that level, the SCRswitches to a relatively high resistance state whereupon the flow ofload current is blocked until the SCR is subsequently retriggered.Therefore when connected to an AC power source an SCR will necessarilycease conducting at the occurrence of a natural current zero.

SCRs are undirectional controlled switches, therefore in an AC powerdistribution system they are nor- .mally connected in an inverseparallel configuration to form a static switch having a pair ofconducting paths (one path conducts positive or forward half cycles ofload current and the other path conducts negative or reverse half cyclesof load current). A control circuit is normally provided for supplyinggate signals to the switch or power SCRs to initiate conduction therein.The control circuit includes means for effectuating load currentinterruption in response to a sensed fault or overcurrent. This may beaccomplished by stopping the supply of gate signals from the controlcircuit, whereupon the switch or the power SCRs would commence blockingload current at the occurrence of the next natural current zero. Itshould be noted that this manner of current interruption may allow thefault current to build up to dangerous levels before the conductingswitch regains its blocking state at the next current zero following thefault currents detection.

In order to provide current interruption capability within a fraction ofa half cycle of the alternating source voltage, means must be providedto force the conducting power SCR off (i.e., return it to its blockingstate). The process of turning off the conducting power SCR is known inthe art as forced commutation or simply commutation. A static switchequipped with commutation means for interrupting current within afraction of a half cycle of the detection of a fault is known as acurrent limiting switch. Such a switch limits the magnitude of the faultcurrent to an acceptable maximum by interrupting the fault current earlyin its half cycle (i.e., before it reaches its peak magnitude).

The commutating means can take a variety of forms which are well knownin the art. One commonly used commutation circuit comprises a chargedcapacitor connected in series with a thyristor (the thyristor is knownin the art as a commutating thyristor, and the capacitor is known as acommutating capacitor). This circuit is connected in shunt across thepower SCR of the static switch. The commutating thyristor is poled inthe same direction as the power SCR and is normally in a nonconductivestate. The commutating capacitor is charged to a predetermined DCvoltage in opposition to the polarity of the power thyristor and isisolated from the power thyristor by the nonconducting commutatingthyristor. When a fault current whose magnitude exceeds a preselectedlevel is detected in the system, the commutating thyristor is triggeredby its control circuit. This allows the charged commutating capacitor todischarge in the reverse direction through the conducting powerthyristor. The commutating capacitor discharge serves to reverse biasthe power thyristor and to drive the current flowing through it belowits holding level, whereupon it turns off (resumes its blocking state).

For high voltage applications an AC switch may comprise plural powerthyristors connected in series with one another to form each conductingpath of the switch, whereby the full voltage imposed on the switch isdivided or shared equally among the individual, lower voltagethyristors. In an AC switch wherein each conducting path is composed ofa pair of serially connected power thyristors it has been suggested thatthe commutation function be provided either via a separate commutationcircuit for each thyristor or via a common commutation circuit for eachconducting path.

Insofar as the latter arrangement is concerned, care should be taken inconstructing the switch to utilize in each path thyristors havingsubstantially identical turn off characteristics. Otherwise the powerthyristor which turns off first will be subjected in the reversedirection to the full voltage remaining on the commutating capacitor,and due to this excessive voltage stress the first off thyristor mayeventually be degraded or permanently damaged. The necessity of matchingturn off characteristics of mass produced thyristors so as to constructa switch that will successfully operate with only a single commutatingcircuit for each conducting path is expensive and time consuming.

By utilizing a separate commutating circuit for each of the seriesedthyristors in a conducting path, the necessity of precisely matchingturn off characteristics is obviated. With this arrangement since thevoltage on each commutating capacitor will not exceed a level that iscompatible with the rating of its individually associated powerthyristor, and even if one thyristor in a path turns off before theother, reverse voltage damage should not occur. For this reason aseparate commutating circuit for each power thyristor in the conducting,path is preferred, although such an arrangement requires extracommutating capacitors and commutating thyristors. From an economicstandpoing it would be desirable to be able to construct a currentlimiting AC switch having both unmatched power thyristors and fewercommutating components.

Accordingly, it is a primary object of my invention to provide arelatively high voltage, current limiting, AC static switch utilizing aminimal number of commutating components while enabling the use ofunmatched power thyristors.

In an AC static switch, either one (but not both) of the inversely poledload current paths may be conducting at the particular moment a fault orovercurrent is detected.

In order to ensure that commutation proceeds effectively if all of thecommutating circuits are actuated at that time, it has been proposed inU. 8. Pat. No. 3,558,983 (Steen) to use decoupling inductors connectedbetween the respective paths and one end of the switch. This minimizesthe forward bias effect on the conducting power thyristors of thedischarge of the commutating capacitors associated with a nonconductingpath. Another approach to ensuring that commutation proceedssuccessfully once it has begun is to actuate only the commutationcircuits associated with the path that was actually conducting. Such adiscriminating approach is shown and claimed in copending U. S. Pat.application Ser. No. 76,446 filed on Sept. 29, 1970 and assigned to thesame assignee as my invention. While this saves the space and cost of atleast the pair of decoupling inductors, it adds the complications andcost of load current direction discriminating and control circuitry.

Accordingly, it is a further object of my invention to provideaseries-thyristor, current-limiting, AC switch including relativelysimple and inexpensive means for ensuring successful commutationnotwithstanding the fact that all of the commutating circuits aresimultaneously actuated.

When thyristors switch from their conducting to their blocking states onthe incidence of a current zero, voltage transients (hereinafterreferred to as switching transients) are generated in the power circuit.In the case of a static switch being commutated off by the discharge ofan associated commutating capacitor, the resulting switching transientappears on the commutating capacitor and is of opposite polarity tothe'voltage to which the capacitor was charged for its commutation duty.If the switching transient is large, damage to the commutating capacitormay result.

Accordingly, it is a further object of my invention to provide improvedmeans coupled to a force-commutated-static switch for suppressingcommutation-induced-voltage transients.

When connected in a power system a static switch may occasionally besubjected to an externally produced voltage surge (e.g., lightningstriking a system conductor) at a time when the power thyristors are intheir nonconducting state. If the surge is severe and if the powersystem lacks surge suppressing circuitry, some of the nonconductingpower thyristors may be damaged.

Accordingly, it is yet a further object of my invention to provide meanscoupled to a forced commutated static switch which is effective forprotecting the switch from externally generated voltage surges.

SUMMARY OF THE INVENTION In accordance with one form of my invention Iprovide a bidirectional current limiting static switch. The switch iscomposed of four power thyristors connected in a bridge circuitincluding two conducting paths. One path, composed of two seriallyconnected power thyristors, is adapted for carrying load current ofpositive polarity and the other path, composed of two other seriallyconnected power thyristors is adapted for carrying load current ofnegative polarity. Commutation means are included in the switch tointerrupt the flow of load current therethrough in response to a stop"signal. The commutation means includes a normally nonconductive switchassociated with each power thyristor and a pair of commutatingcapacitors. Each of said capacitors is associated with one powerthyristor of one path and one power thyristor of the other path. Whenarranged in this manner, each capacitor is adapted forsupplying'commutating energy to two 7 BRIEF DESCRIPTION OF THE DRAWINGThis invention will be better understood and its various objects andadvantages will be more fully appreciated from the following descriptiontaken in conjunction with the accompanying drawing which is.a schematicdiagram of a power system utilizing a static switch in accordance withmy invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION As canbe seen an alternating voltage source 1 is arranged to supply electricpower to a load 3. In order to initiate or terminate the flow of currenttothe load, a static circuit breaker 2 is provided between the sourceand the load. This circuit breaker includes a solid state or staticswitch 4. Although only a single-phase switch is shown, it will beunderstood that two more duplicate switches would be used in a typical3-phase static breaker. In order to control conduction of the staticswitch, i.e., initiate or terminate current conduction therein, thebreaker also includes a power control circuit 5 having two states ormodes, namely ON and OFF." When control circuit 5 is actuated from itsON to its OFF states the static switch interrupts the flow of current tothe load. Further, the breaker includes commutation means to forcecommutate the static switch in high-speed response to the detection of afault current by an overcurrent detecting circuit 6. The commutationmeans are controlled by a commutation control circuit 7.

The static switch 4 comprises four power thyristors 4a, 4b, 4c and 411connected in a bridge circuit to provide a switch having a bidirectionalload current conduction capability. As can be seen the anode ofthyristor 4a is connected to the cathode of thyristor 4c at one ACterminal, AC1, of the switch. In a similar manner the anode of thyristor4d is connected to the cathode of thyristor 4b at the other AC terminal,AC2, of the switch. The cathodes of thyristors 4a and 4d are connectedtogether at one DC terminal, DCl, of the switch and the anodes ofthyristors 4c and 4b are connected at the other DC terminal, DC2, of theswitch. When arranged in this manner thyristors 4a and 4b form oneconducting path and thyristors 4c and 4d form a second conducting path.As should be appreciated one conducting path is adapted for conductingcurrent of positive polarity to the load and the other path is adaptedfor conducting current of negative polarity thereto. Since each pathincludes two serially connected power thyristors, the switch isparticularly adapted for use in high voltage circuit breakingapplications. For higher current ratings, the individual thyristorsshown can be respectively paralleled by duplicate devices which operatein unison therewith. Respective voltage dividing bypass circuits areconnected in shunt with each serially connected power thyristor. Suchcircuits are sometimes referred to as snubber circuits and commonlyinclude a resistor connected in series with a capacitor. For the sake ofdrawing simplicity the snubber circuits included in the static switch 4are not shown.

Control circuit 5, in its ON mode, provides suitable gate signals to thepower thyristors making up switch 4 to render the switch conductive,whereupon load current is able to flow between the source 1 and the load3. In its OFF mode no gate signals are provided by control circuit 6 toany of the switch power thyristors. Hence, when control circuit 6 is inthis mode the static switch 4 blocks the flow of load current.

Static circuit breaker 2 is of the current limiting type and istherefore equipped with means for rapidly forcing all conducting powerthyristors OFF in response to a sensed fault. That means includes a pairof commutation circuits 8 and 9. Commutation circuit 8 includes acapacitor 8a and an inductor 8b and is connected at one end to a DCterminal of the bridge circuit. In a similar manner commutation circuit9 includes a capacitor 9a and an inductor 9b and is connected at one endto the other DC terminal of the bridge circuit. A normally nonconductivecommutating thyristor 10a is connected between one AC terminal of thebridge circuit and one end of the commutation circuit 8. A normallynonconductive commutating thyristor 10d is connected between the otherAC terminal of the bridge circuit and the end of the commutating circuit8. In a similar manner a normally nonconductive commutating thyristor10c is connected between one AC terminal of the bridge circuit and oneend of the commutation circuit 9. Another nonconductive commutatingthyristor 10b is connected between the other AC terminal of the bridgeand one end of the commutation circuit 9. When connected in this mannereach power thyristor has associated with it a similarly poledcommutating thyristor. Capacitor 8a of commutating circuit 8 andcapacitor 9a of commutating circuit 9 are charged to a DC voltage level,the polarity of which is shown. Although not shown in the drawing, useof a precharging scheme such as that claimed in U. S. Pat. No.3,098,949-Goldberg is contemplated.

When a fault occurs, current flowing in the switch increases abnormally.When the magnitude of fault current attains a preselected level,overcurrent detecting circuit 6 is activated and immediately provides astop signal to the power control circuit 5 and to the commutationcontrol circuit 7. Upon receipt of a stop signal, control circuit 5ceases producing gate signals for the power thyristors. In response tothe same event, commutation control circuit 7 is arranged to supply agate signal to all of the commutating thyristors.

If the fault current is large, irrespective of which conducting path iscarrying that current, a voltage of the polarity shown will appear oninductor 11 and will be of sufficient magnitude to reverse bias thecommutating thyristors associated with the nonconducting powerthyristors. This action will prevent those commutating thyristors frombecoming conductive notwithstanding the application of the triggersignals thereto from control circuit 7. For example, assume that faultcurrent is flowing through the conductive path made up of powerthyristor 4a, inductor 1 1 and power thyristor 4b and attains apreselected level. Overcurrent detecting circuit 6 feeds a stop signalto power control circuit 5 and commutation control circuit 7, and thelatter circuit provides trigger signals to the gates of all of thecommutating thyristors. If the fault current is high enough, the voltageappearing on inductor 11 will reverse bias commutating thyristor 10d and100 notwithstanding the fact that the commutating capacitor connected toeach of them will tend to forward bias them. Commutating thyristors 10aand 10b will now begin conducting, and commutation of the conductingpower thyristors 4a and 4b will occur in the following manner: theenergy stored in commutating capacitor 8a will be enabled to flow in thereverse direction through power thyristor 4a via conducting commutatingthyristor 10a. In a similar manner the energy stored in commutatingcapacitor will be enabled to flow in the reverse direction through powerthyristor 4b via commutating thyristor 10b. The reverse current flowingthrough the power thyristors quenches load current conduction therein.Due to the fact that each commutating circuit contains an inductor(i.e., commutating circuit 8 includes inductor8a and commutating circuit9 includes inductor 9a) the current flowing through the conductingcommutating thyristors is oscillatory in nature and at the occurrenceage appearing on inductor 11 as fault current flows therethrough is afunction of the rate of rise of that current (di/dt). In certainsituations the di/dt may be relatively small so that the voltageappearing on inductor 11 may be insufficient to reverse bias thecommutating thyristors associated with the nonconducting powerthyristors (i.e., the voltage appearing on inductor 11 may be less thanthe voltage on capacitors 8a and 9a). Accordingly, all the commutatingthyristors may begin conducting upon receipt of trigger signals from thecommutation control circuit 7. In the interest of successful commutationof the fault current carrying power thyristors, it is important that theenergy discharging from the capacitors of the commutating circuits bedelivered in the reverse direction through the conducting powerthyristors. In that regard it is of utmost importance to prevent thecommutating capacitor discharge current from circulating around circuitpaths including conducting commutating thyristors but excluding theconducting power thyristors (one such path consists of commutatingthyristor 10a, commutating capacitor 8a, commutating inductor 8b,current limiting inductor 11, commutating inductor 9b, commutatingcapacitor 9a, and commutating thyristor 10c, and a second such pathconsists of commutating thyristor 10d, commutating capacitor 8a,commutating inductor 8b, current limiting inductor 11, commutatinginductor 9b, commutating capacitor 9a and commutating thyristor 10b).

The current limiting inductor 11 serves to prevent the discharge currentfrom the commutating capacitors from circulating in those paths at leastuntil the fault current carrying power thyristors have turned off. In sodoing the current limiting inductor ensures that commutation of thefault current carrying power thyristors can proceed to a successfulconclusion irrespective of whether or not the commutating thyristorsassociated with a nonconducting power thyristor become conductive. Theoperation of the inductor in this respect will be described withreference to the commutation of power thyristor 4a (assuming that thefault current is flowing through thyristors 4a and 4b and that all ofthe commutating thyristors turn on upon being triggered by thecommutation control circuit 7). When commutating thyristor 10a istriggered, current begins flowing from commutating capacitor 8a in thereverse direction through power thyristor 4a via the path consisting ofthe conducting commutating thyristor 10a, commutating capacitor 8a,commutating inductor 8b and power thyristor 4a. At the same time thecurrent flowing through conducting power thyristor 4a in the forwarddirection includes not only the fault current but also includes currentfrom discharging commutating capacitor 9a. The'latter current is enabledto flow via conducting commutating thyristor 100 (it flows'through thepath consisting of commutating thyristor 100, power thyristor 4a,current limiting inductor 11, commutating inductor 9b and commutatingcapacitor 9a). The inductance of each commutating inductor 8a and 9a issmaller than the inductance of current limiting inductor 11 (e.g.,inductor 11 has approximately 10 times the inductance of inductor 8a andinductor 9a).

Therefore, the current flowing in the reverse direction through theconducting power thyristor 4a (i.e., the current from commutatingcapacitor 8a) encounters less impedance than the current flowing in theforward direction therethrough (i.e., the relatively slow rising faultcurrent and the current from commutating capacitor 9a which musttraverse the relatively large current limiting inductor 11), and the netforward cur rent is rapidly reduced. This action results in thequenching of current flow through power thyristor 4a. Once powerthyristor 4a ceases conducting the remaining discharge current from thecommutating capacitors can flow through the path including commutatingthyristors 10a and 100, current limiting inductor l1 andcommutating'circuits 8 and 9. Due to the inductance in this path thecurrent is oscillatory in nature and at the occurrence of the currentzero the commutating thyristors cease conducting and the flow of faultcur- I rent is terminated. It should be appreciated that in the aboveexample power thyristor 4b would be commutated off in a similar manner.Whether all commutating thyristors turn on or only those associated withthe fault current conducting power thyristors turn on, the commutationsequence is completed within a few hundred microseconds from the timethe fault of a preselected magnitude is sensed. Therefore, the faultcurrent which is permitted to flow can be limited to an acceptablemagnitude, (i.e., well below the available peak fault current magnitude)by the very rapid response of the static circuit breaker 2. v

The static switch 2 also includes common circuitry effective forprotecting the commutating capacitors fromswitching-created-voltage-surges and for protecting nonconducting powerthyristors from-externally produced voltage surges. 1

As is known, when a power thyristor switches from its conducting to itsnonconducting state, a voltage transient is generated. This transientwill appear across the commutating capacitor associated with the powerthyristor which is turning off and will be of opposite polarity to thenormal voltage on the commutating capacitor. If the magnitude of theswitching transient is large, damage to the commutating capacitor or tothe power thyristor may result.

As can be seen in the drawing a surge suppressing circuit 12 isconnected between the common points of commutating thyristors 10a and10d and the common point of commutating thyristors 10c and 10b. Thecircuit 12 includes a unipolarity conducting element or diode connectedin series with an energy dissipating element or resistor, 12b. The diodeis poled in opposition to the polarity of the voltage normally appearingon the charged commutating capacitors so that the energy stored in thosecapacitors will not be diverted into the surge suppressing circuitduring the process of commutation.

Operation of the surge suppressing circuit 12 will be considered belowwith regard to the commutation of power thyristor 4a. Absent the surgesuppressing circuit, as thyristor 4a turns off (i.e., regains itsblocking state), a switching voltage transient would appear on thedischarging commutating capacitor 8a with reverse polarity compared tothe initial charge. The transient is generated by the fault currentswitching to the conducting commutating thyristor. In flowing throughthe conducting commutating thyristor, the fault current tends to buildup charge on the capacitor in the reverse direction. However, my surgesuppressing circuit 12 effectively limits such a build-up of reversevoltage, and any excessive energy in the switching transient, instead ofoverstressing the commutating capacitor, will pass through diode 12a toresistor 12b to be safely dissipated therein. 1

The surge suppressing circuit 12 may also be used to protect the switchspower thyristors from externally generated voltage surges which mightappear on the source side of the switch when it is in its OFF mode. Aspreviously noted when the switch 2 is in its OFF mode all of theswitches thyristors are in their blocking or nonconductive states. Toeffectuate surge suppression in this instance, I insert the circuit 12into the path through which the surge will pass by triggering all of thecommutating thyristors into conduction in response to the detection ofthe surge. To that end a surge detection means 13 is provided in thestatic circuit breaker coupled to the commutation control circuit 7 andthe power control circuit 5. Such means causes the commutation controlcircuit to provide trigger signals to the commutating thyristors inresponse to the detection of a line surge above a preselected magnitudeat a time when the power control circuit is in its OFF mode. Insofar asexternal line surges are concerned operation of the surge suppressingcircuit is as follows. Upon being triggered, the commutating thyristorsbegin conducting thereby forming two local currentpaths through whichthe energy stored in the commutating capacitors can circulate. One ofsuch paths includes commutating thyristor a, commutating capacitor 80,commutating inductor 8b, current limiting inductor 1 1, commutatinginductor 9b, commutating capacitor 90 and commutating thyristor 10c. Theother local path includes commutating thyristor 10d, commutatingcapacitor 8a, commutating inductor 8b, current limiting inductor ll,commutating inductor 9b, commutating capacitor 9a and commutatingthyristor 10b. The circulation of the capacitor discharge current aroundboth local paths effectively transfers the commutation capacitor energyto the current limiting inductor in the polarity shown. Furthermore,once all four commutating thyristors are conducting and capacitordischarge currents begin circulating in the local paths, the voltageappearing across the AC terminals of the switch is effectively zero(neglecting the slight voltage drops across the conducting commutatingthyristors). Accordingly, the nonconducting power thyristors of theswitch do not have to withstand the voltage surge and instead the surgeis passed to the load.

Since each of the local paths includes inductance and capacitance, thecurrent circulating therethrough is oscillatory in nature. At theoccurrence of the natural current zero the commutating thyristors ceaseconducting and the circulating currents cease flowing. Now the energystored in the current limiting inductor will begin discharging into thesurge suppressing circuit where it is safely dissipated in resistor 12b.

It should be noted that in protecting the nonconducting power thyristorsfrom external line surges the conduction of the commutating thyristorsnecessarily passes the surge to the load. Accordingly it is assumed thatwhen a static circuit breaker is provided with such external voltagesurge protecting means the load should be capable of absorbing suchvoltage surges, and if it is not capable of absorbing such surgesadditional protective circuitry (e.g., lightning arrestor s) should beprovided.

While I have shown and described a particular embodiment of myinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from myinvention in its broader aspects; and I, therefore, intend herein tocover all such changes and modifications as fall within the true spiritand scope of my invention.

What 1 claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. For use in a power system including an AC power source and a load, abidirectional switch for conducting current between the source and theload and for interrupting the flow of such current upon command, saidswitch comprising:

a. first, second, third and fourth thyristors connected to form a bridgecircuit having a pair of AC terminals and first and second DC terminals,each of said thyristors including an anode and a cathode, the cathodesof said first and third thyristorsbeing connected in common to saidfirst DC terminal, and the anodes of said second and fourth thyristorsbeing connected in common to said second DC terminal;

b. current limiting impedance means connected between said DC terminals,said impedance means together with said first and second thyristorsforming a first load current conducting path between said AC'terminals,said impedance means together with said third and fourth thyristorsforming a second load current conducting path between said AC terminals;

0. first energy storage means connected between said first DC terminalsand a first point; second energy storage means connected between saidsecond DC terminal and a second point; and

e. first, second, third, and fourth controlled switch means adapted tobe triggered upon command, said first and third switch means beingconnected to one another between said AC terminals with their juncturecomprising said first point, and said second and fourth switch meansbeing connected to one another between said AC terminals with theirjuncture comprising said second point.

2. The bidirectional switch as specified in claim 1 wherein said currentlimiting impedance means is an inductor.

3. The bidirectional switch as specified in claim 1 wherein each of saidenergy storage means is a capacitOl'.

4. The bidirectional switch as specified in claim 3 wherein said currentlimiting impedance means is an inductor.

5. The bidirectional switch as specified in claim 3 wherein each of saidcontrolled switch means is a thyristor.

6. A bidirectional switch as specified in claim 1 wherein a surgesuppressing circuit is connected between said first and said secondpoints.

7. A bidirectional switch as specified in claim 6 wherein said powersystem includes means for detectdissipating element.

9. Bidirectional switch as specified in claim 8 wherein said unipolarityconducting element is a diode and wherein said energy dissipatingelement is a resistor.

1. For use in a power system including an AC power source and a load, abidirectional switch for conducting current between the source and theload and for interrupting the flow of such current upon command, saidswitch comprising: a. first, second, third and fourth thyristorsconnected to form a bridge circuit having a pair of AC terminals andfirst and second DC terminals, each of said thyristors including ananode and a cathode, the cathodes of said first and third thyristorsbeing connected in common to said first DC terminal, and the anodes ofsaid second and fourth thyristors being connected in common to saidsecond DC terminal; b. current limiting impedance means connectedbetween said DC terminals, said impedance means together with said firstand second thyristors forming a first load current conducting pathbetween said AC terminals, said impedance means together with said thirdand fourth thyristors forming a second load current conducting pathbetween said AC terminals; c. first energy storage means connectedbetween said first DC terminals and a first point; d. second energystorage means connected between said second DC terminal and a secondpoint; and e. first, second, third, and fourth controlled switch meansadapted to be triggered upon command, said first and third switch meansbeing connected to one another between said AC terminals with theirjuncture comprising said first point, and said second and fourth switchmeans being connected to one another between said AC terminals withtheir juncture comprising said second point.
 2. The bidirectional switchas specified in claim 1 wherein said current limiting impedance means isan inductor.
 3. The bidirectional switch as specified in claim 1 whereineach of said energy storage means is a capacitor.
 4. The bidirectionalswitch as specified in claim 3 wherein said current limiting impedancemeans is an inductor.
 5. The bidirectional switch as specified in claim3 wherein each of said controlled switch means is a thyristor.
 6. Abidirectional switch as specified in claim 1 wherein a surge suppressingcircuit is connected between said first and said second points.
 7. Abidirectional switch as specified in claim 6 wherein said power systemincludes means for detecting the presence of a voltage surge at a timewhen said bridge thyristors are nonconductive and for rendering saidcontrolled switches conductive in response thereto.
 8. The bidirectionalswitch as specified in claim 6 wherein said surge suppressing circuitcomprises a unipolarity conducting element in series with an energydissipating element.
 9. Bidirectional switch as specified in claim 8wherein said unipolarity conducting element is a diode and wherein saidenergy dissipating element is a resistor.